Forget the tony suburbs around San Francisco, Los Angeles and New York.

Little Los Alamos County is the fifth wealthiest county in the nation as measured by household income, U.S. Census Bureau data show.

In fact, Los Alamos can celebrate being the wealthiest county west of the Mississippi River based on household income data the U.S. Census Bureau collected from Jan. 1, 2005 to Dec. 31, 2009. That's quite an accomplishment for an area more apt to compete for the highest number of Nobel laureates to have once worked in the area, thanks to Los Alamos National Laboratory.

According to the Census Bureau's American Community Survey, Los Alamos County's median household income is $100,432. Median is the midway point between households with higher and lower incomes, meaning half the households in Los Alamos County earn more than that and half earn less.

Los Alamos County's designation as one of the wealthiest areas in the nation is all the more striking because it sits like an island of affluence in New Mexico, which is one of the poorest states in the nation. New Mexico's median household income is $42,742.

Sure, Santa Fe has a reputation as a playground for the wealthy. But at $52,923, Santa Fe County's household income is only about half of Los Alamos County's. Sandoval County, meanwhile, has the second-highest median household income in New Mexico. But at $56,703, Sandoval County has some catching up to do if it ever wants to compete with its next-door neighbor.

Los Alamos County, like many areas, has lost a bit of economic ground over the past decade because of the economic slowdown, but not much. The 2000 Census estimated Los Alamos County's median household income at $101,687.

The county rankings came from 2005 to 2009 estimates of annual household income in each county of the nation, provided by the U.S. Census' American Community Survey. The estimates are in 2009 inflation-adjusted U.S. dollars.

It's no secret that Los Alamos is an enclave of the generally affluent.

A few years ago, Kiplinger's Personal Finance Magazine designated the city of Los Alamos as having one of the highest per capita population of millionaires. It was partly a function of Los Alamos's size. With about 11,900 residents, the city makes up just over half of the county's population of 18,400. That magazine found one in five Los Alamos households had a net worth of $1 million dollars or more, excluding the value of their primary residence.

A big part of the area's wealth can be attributed to LANL, the area's largest employer, which provides thousands of high-paying jobs.

More than half of Los Alamos County's 7,800 households reported income and benefits of $100,000 or more, according to the American Community Survey, while only 1.9 percent of the county's families were living at poverty level or lower.

The counties that beat out Los Alamos for the top four spots are in descending order are: Loudoun County, Va., with a median household income of $112,021; Fairfax County, Va., $104,259; Hunterdon County, N.J., $102,500; and Howard County, Md., $101,003.

Los Alamos County barely won fifth place, edging out the only other Western entry in the Top 10, Douglas County, Colo. Douglas County, a few hundred miles up Interstate 25 between Denver and Colorado Springs, had a median household income of $99,522, census data show.

Contact Trip Jennings at 986-3050 or at tjennings@sfnewmexican.com.

MEDIAN HOUSEHOLD INCOMES

• Los Alamos County: $100,423

• Santa Fe County: $52,923

• Rio Arriba County: $42,514

• New Mexico overall: $42,742